Like others have said, no your not supposed to. Can you make it an outlet location versus a blank? That would be my preference if you cant run an entire new wire.
I have burried a box once but it was the only way to do the project w/o significant major headaches. Yea yea think of that guy trying to troubleshoot something down the road and what a pain it would be. Sorry if thats they guy reading this someday. I did use a box, a cover, wire nuts and electrical tape. I suppose I could have also soldered and used heat shrink tape for an even better connection but oh well. I don't live there any more. Blank or outlet were not an option (It was under a sub floor, under carpet)